The present invention relates to a system for preventing trouble caused by overheating of an electro-magnetic powder clutch for automobiles.
An electro-magnetic powder clutch is used for coupling a crankshaft of an engine with an input shaft of a transmission. Such electro-magnetic clutch are disclosed, for example, in U.S. Pat. Nos. 3,073,422 and 3,239,038. The electro-magnetic powder clutch comprises an annular drive member secured to the crankshaft of the engine, a magnetizing coil provided in the drive member, a driven member secured to the input shaft of the transmission and spaced from the drive member, and powder provided in the clutch. When the first gear or second gear is selected and the accelerator pedal is depressed, electric current flows through the magnetizing coil to magnetize the drive member. The powder is aggregated in the gap by the magnetic flux, so that the driven member is coupled member through the powder. Thus, the output of the engine may be transmitted to the transmission.
The clutch current in the magnetizing coil progressively increases depending upon the degree of depression of the accelerator pedal, while the clutch slips between the drive member and the driven member. Thus, the vehicle may be smoothly and easily started by depressing the accelerator pedal without operating a clutch pedal. The clutch slips continuously until the clutch torque reaches a value equal to the engine torque, namely, the stall torque. Stall speed at the stall torque in each transmission gear speed depends on the rotational speed of the engine. If the vehicle is started in the high speed stage such as 3rd or 4th speed, the clutch slips over a wide range as the engine speed increases to a high speed. As a result, the temperature of the clutch is excessively raised, which causes trouble by overheating of the clutch.
In order to prevent such overheating, conventionally, the clutch is so arranged that if the vehicle is started in 3rd and 4th speeds, the clutch increases so that the clutch is suddenly connected before the engine torque reaches a sufficient value in order to be able to drive the transmission load. Thus, the vehicle cannot be started due to the development of jolts or stalling of the engine.
On the other hand, the clutch is so arranged to permit a 2nd speed start in order to simplify the starting operation. However, if the 2nd speed start is repeated in particular circumstances such as in a traffic jam, the clutch is overheated by the slipping so that the insulation for the coil burns. Further, the grease in the bearings in the clutch is softened by the overheating so that the grease leaks therefrom, thereby decreasing the durability of the bearings. In addition, the grease enters the powder chamber of the clutch, promoting slipping of the clutch thereby causing damage on the clutch.